How can a texture converted to be tileable (seamless)?
Blender's UV / Image does not provide such a functionality (AFAIK). This can easily achieved by GIMP.

After copying a region from the photo into a new image (preferably with dimensions as a power of two e.g. 1024x1024 pixel)

Execute from main menu: Filters / Map / Make Seamless



Result:



The terrain could be randomized (Edit Mode menu: Mesh / Transform / Randomize)




Related: 


Unwanted patterns in texture from repeated, seamless image
Why should texture resolutions be powers of 2?
https://gamedev.stackexchange.com/questions/26187/why-are-textures-always-square-powers-of-two-what-if-they-arent